You can choose not to tell/educate your U18 child about the annual gift, but I don't think that's a good idea. Best to actually give them some reponsibility in their future and not try to control their lives indefinitely.
You could tell them about it at age 34, because that's the average age of a gambling addict entering treatment, or at 43, because that the median age for individuals entering alcohol treatment in Ireland. I know older people again that still can't manage money because no one ever taught them how. There's no guarantee that the 50/60 year old (or their spouse) won't go bananas with an inheritance. No one can control the outcome.
Again, availing of this specific annual exemption should really only be the concern of those that know now that the CAT threshold will be exceeded.